Rubber tires are composite materials consisting of different rubber polymers, which have been blended with a variety of additives including sulphur and carbon blacks. During manufacture, polymers are layered with reinforcing fabrics and cords made from fibreglass, polyester and steel and are assembled and compressed into a basic shape called a green tire. This green tire is then cured at a specified temperature for a specified time.
The curing process, known as vulcanization, causes sulphur to crosslink with the various rubber polymer chains to form a three-dimensional network that results in a thermoset polymer. Thermoset polymers are contrasted with thermoplastic polymers such as polyethylene, polystyrene and polyvinylchloride which can be softened and reshaped by the use of heat or by dissolving the polymer in a suitable solvent. Whereas a thermoplastic polymer can be dissolved within a solvent that enables the polymer to be cast as a polymer solution with the solvent subsequently evaporated to re-form a solid polymer, thermoset polymers are infusible and insoluble and cannot be reshaped once formed and cured.
As a result, with used vulcanized rubber products, and particularly products such as worn rubber tires, reuse and recycling is problematic as the vulcanized rubber cannot be fused and reshaped nor dissolved in a solvent for recasting into a new shape as is possible with thermoplastic polymers.
In past attempts to re-use or recycle used rubber tires, reshaping of used rubber tires into new products has been accomplished by cutting or grinding the rubber tire to produce strips of rubber or crumb of a suitable size that can be used as building blocks or aggregate which, when combined with bonding agents such as resins, can be formed into new shapes. While a variety of innovative products have been created by this methodology, this approach has provided only a limited outlet for recycling used tires.
Other methods to efficiently dispose of or re-use discarded tires have also been explored. For example, used rubber tires have been shredded and reused to make other products such as rubber mats, padding materials, or asphalt additives. In addition, processes for co-recycling rubber tires with other materials have been described (see for example U.S. Pat. No. 5,389,691). Used tires have also been used as thermal fuel which is generally not desirable for environmental reasons due to the atmospheric emissions that result from burning vulcanized rubber. Exceptions to this include use of tires to fuel cement kilns and for carbon addition to steel making.
Further still, there have been attempts to soften rubber tires using high temperatures and/or pressures. For example, U.S. Pat. No. 5,672,630 to Mouri describes a method to soften vulcanized rubber by kneading it with unvulcanized new rubber at high temperatures. However, this process does not result in a truly devulcanized rubber product.
Thus, despite the many recycling/reuse initiatives, it is estimated that each year in the U.S., there are upwards of 200 million tires that are stranded without an end use or that are used as thermal fuel.
As a result, and due to the ever-increasing numbers of tires and the growing awareness of environmental issues, it remains desirable to find methods to devulcanize vulcanized rubber such that the devulcanized rubber can be cast into new products including new tires. More specifically, it has been desirable to develop devulcanization processes that remove the sulphur crosslinks within the vulcanized rubber, resulting in breakdown of the three-dimensional polymer network and the production of polymers that can be fused and reshaped into new products such as tires.
Past methodologies to devulcanize rubber have included various approaches, some of which are discussed below. For example, U.S. Pat. No. 5,891,926 to Hunt describes a process including heating vulcanized rubber in the presence of 2-butanol under high pressure.
Other processes claim to remove or reduce the sulphur crosslinking within rubber tires. These processes include microwave treatment, ultrasonic treatment, milling with additives, and chemical processing. These approaches to devulcanization of rubber tires have, however, proved difficult and inefficient. Common problems include poor removal of crosslinks, thermal cracking which degrades the rubber polymers, and high temperature and pressure requirements.
Most importantly, however, the major limitation in past devulcanization techniques is overcoming the inherent mass transfer limitations in reacting the solid rubber crumb with an agent effective in removing the sulphur crosslinks. In other words, as the devulcanization is initiated at the outside surface of the rubber crumb, the reaction is very slow unless the size of the crumb is exceedingly small. Attaining small rubber crumb is costly and the process can mechanically degrade the rubber polymers. Furthermore, the slow reaction rate also leads to thermal cracking of the polymer, which reduces the molar mass of the devulcanized polymer, thereby degrading the mechanical properties of the devulcanized rubber, and also producing light hydrocarbon gaseous products. Thermal cracking may also lead to condensation reactions, which increase the polymer molar mass and can lead to the formation of solid coke which can be detrimental to the properties of new materials made from the devulcanized rubber
In other examples, U.S. Pat. Nos. 5,798,394 and 5,602,186 to Myers describe a method to devulcanize rubber crumb using metallic sodium. In this method, the rubber crumb is first slurried with a solvent such as cyclohexane or toluene to swell the rubber crumb. Alkali metal is then added as the devulcanization agent. The reaction is carried out in the absence of oxygen and in the presence of hydrogen, requiring a two-fold stoichiometric excess of sodium with respect to sulphur content of the rubber. Temperatures and pressures sufficient to form molten sodium are used. Moreover, the reaction system includes four phases with inherent mass transfer limitations. In this process, reactions of the sulphur crosslinks with the sodium metal, which is a liquid at reaction conditions, can only occur at the external surface. There is no efficient mass transfer of molten sodium into the three-dimensional polymer network where it can react with the sulphur crosslinks.
U.S. Pat. No. 4,161,464 to Nicholas describes the devulcanization of rubber particles following swelling of the rubber particles with an organic solvent containing a dissolved onium salt. This slurry is then contacted by an alkali solution. The onium salt exchanges its anion for a hydroxyl anion at the interface between the organic solution and the aqueous alkali solution. The onium salt, carrying the hydroxyl anion, can diffuse to the crumb and within the crumb, the hydroxyl anion can react with sulphur. The swelling of the rubber by the organic solvent is said to facilitate permeation by onium hydroxide pairs. Once the hydroxyl anion reacts, the onium salt must re-diffuse to the organic-aqueous interface and re-exchange its anion for another hydroxyl anion prior to another diffusion and reaction cycle
U.S. Pat. No. 4,426,459 to Watabe also discusses swelling vulcanized rubber with a solvent, followed by reaction with an organic hydroperoxide, a salt or organometallic compound of an element taken from the first transition series of the periodic table of elements and a strong base. Treatment is carried out in an aerobic environment at temperatures between 0° C. and 100° C. U.S. Pat. No. 5,770,632 to Sekhar describes a process in which one or more so-called delinking accelerators is combined with zinc oxide to delink sulphur-cured rubber and open up the vulcanized network.
U.S. Pat. No. 5,275,948 to Straube reports the utilization of chemolithotropic microbes in an aerobic environment to release sulphur from vulcanized rubber as elemental sulphur and sulphuric acid. The rubber crumb is preferably finely ground to 50-350 microns. Straube teaches that it is sufficient to devulcanize the rubber crumb in this way to a depth of a few micrometers, i.e, the rubber crumb is devulcanized at the surface. Similarly, Romine and Snowden, in U.S. Pat. No. 5,597,851, teach the use of thiophyllic microbes or enzymes from thiophyllic microbes for conversion of sulphur crosslinks in vulcanized rubber to sulphoxides and sulphones. Romine also teaches that only the sulphur crosslinks exposed on the surface of the rubber crumb need be converted in this manner. The approach proposed by Romine provides a surface modified rubber crumb. Neither the approach taught by Romine nor Straube would lead to products that could be fused and reshaped into new rubber products in the same manner as the original unvulcanized rubber polymers.
The various devulcanization processes have drawbacks limiting their use on a large scale. Some chemical and biochemical processes appear capable of devulcanizing rubber but either devulcanize only a superficial layer on the rubber crumb or are inefficient. The inefficiencies may be due to significant mass transfer limitations caused by the reaction occurring only on the external surface of the crumb or by reactants that must be continuously transferred from solution into the solid crumb. An additional concern with some of these chemical processes is that they require relatively high temperatures. The use of high temperatures combined with mass transfer limited reactions lead to greater opportunity for thermal degradation reactions to occur. Thermally degrading reactions can shorten the length of rubber polymer chains or otherwise change their chemical structures such that their mechanical properties are adversely affected, thus limiting their usage in new rubber products.
In other technologies unrelated to devulcanization, certain compounds or catalysts have been used to remove or modify sulphur linkages within various chemical entities. Such a reaction is described in U.S. Pat. No. 5,578,197 to Cyr, which discloses the addition of an oil-soluble metal compound to petroleum feedstock, which under prescribed conditions is converted to a metal sulphide catalyst. The catalyst, in the presence of hydrogen, is useful for hydrocracking the feedstock and removing sulphur as hydrogen sulphide. Other such processes are well known in the petroleum refining industry where sulphur removal is a requirement for converting crude oil to consumer products such as fuels and lube oils
In view of the foregoing problems, it is therefore, desirable to provide improved methodologies to efficiently and effectively devulcanize used rubber to enable its effective re-use.